Backsight elevation check values
Hi Gents,
I??m not sure if this issue has been covered else where, but I have looked high and low.
I have an issue when ever I set up on my foresight and check back to my backsight I??m constantly getting up to 10mm vertical difference over only 100m.
I have taken direct and reverse readings when foresighting the point in.
I performed the check and adjust routine only 2 days ago.
Im using a 5? Ts 15 , Leica round prisms, on a Leica pole and bipods, Leica GHM007 tape with tribrach hook.
It??s been driving me crazy trying to find the issue.
Cheers
- 25 Replies
Have you measured (with a tape measure) the actual pole heights to make sure they are what the pole says it is, or what you assume they are?
Some instruments require a regular vertical circle calibration. First check, is to check your backsight with both faces, same as setting your foresight. Vertical circles are generally no locked to the EDM base but instead have gravimetric base with electronic calibration.
Paul in PA
Just to eliminate the obvious…
Leica instruments do not measure to the center of the scope for height. It has a special braket and tape to measure down.
Your IH would be higher by roughly a tenth or two (memory is foggy) if you measured to the center of the scope
Leica instruments do not measure to the center of the scope for height.
??
You say you have 10mm error at 100m. D0 you have 20mm at 200m? or is it the same 10mm?
The number 1 cause of elevation issues of this type are in the measuring up. Of both the instrument and the target. It seems to me that, with the Leicas, the height hook thing in the software has a place to enter the tape reading, then a constant offset from the height hook mark to the center of the instrument. Perhaps that offset is messed up somehow?
The number 2 cause is collimation. You seem to have addressed that.
Number 3 is the elevations of the points themselves. Just how were these elevations determined?
Steve,
As Mr. Oklahoma pointed out, and I’m sure you have already done, the first thing I would do is verify the height of the two points. As for using the GHM007 tape, it is really easy to read to the wrong cm. I’ve been using this things for going on 20 years now and still find myself miss reading them on the rare occasion (that’s why I always measure twice). At a quick glance I sometimes pick the higher of the two cm readings visible. Also, you really have to make sure the tape is setting flat in the tribrach hook. Sometimes is snags on the top and that will give you a high reading as well. The gradient of GHM007 is corrected to read the true HI to the vertical axis of the scope and thus does not require any offset to be entered. One thing you might verify is that you are actually using a GHM007, Leica make the same tape calibrated for the C10 & P50 which looks identical. Mr. OK is thinking about the GNSS tape hook which requires a 0.360 m offset to get the true HARP of the antenna. For that I do have a couple of Leica laser plumets which have the tape hook hole machined about 10mm off.
Two things to check on the instrument side. First, do you have an offset value set in the TS15? You have the option of resetting the offset to 0 after each shot or leaving it until you change it. This offset is applied when you check a point. Second, when you did the field calibration, did you do a vertical index calibration? It is a second adjustment after the culmination calibration.
Good luck
Are you sighting the center of the prism mount or the center of the prism pivot point on the holder.
The correct point to site is the center of the pivot point.
0.02
Are you measuring the slant height of the instrument, then correcting for the vertical height?
My Leica pole point wore down over time causing me similar problems a few years ago. So now when the rod height reads 6.00 I know it is really more like 5.98, I hope this helps.
How are your vertical closures? I have the same issue with my TS12 but it consistently closes well vertically (around .01′) despite the high backsight residuals.
Yes, I measured the pole before I started. The pole tip is a little worn, so it read 1.298m
I ran the tilted axis routine 5 days ago. And it got used once for the week for about 30mins. I did notice that the delta height difference when foresighting point in was about 8mm different in each face.
I’ve always used a normal tape measure in the past, and measured to the little dot on the side of the unit. I have recently purchased the genuine Leica GHM007 tape measure & GHT196 tribrach hook. I hoped that is was the issue, but the difference in heights still exist.
That’s the dot I measure to. Ive compared the Leica tape against a couple normal tapes, and there’s 4mm difference in measurement . I assume that’s because the Leica tape is calibrated for slope.
I’d have to tested that out, I usually try not to exceed 100m for traverse legs because of this height issue. From memory I shot a longer backsight a month ago, and the backsight height check was around 20mm + out
The Leica tape uses a standard hight offset based on the tribrach with the instrument on top. Everything used is Leica brand. I’ll re-read the Leica tape instructions to make sure I haven’t missed something.
The setup point elevation is just an arbitrary RL100, I then foresight the new point. Then setup on foresight, backsight to original setup point. = 10mm height difference on backsight check
I have ran a level loop over setup points. The TS is missing the elevation by around 10mm. The Leica tapes were tricky to read at the start. But I have been triple checking reading. I agree about the snagging hook, but the tape tip is seated down correctly in hook. I checked, it is a GHM007 tape, and not the gnss version. The offset values reset after storing point. Yes I ran the tilted axis routine after the culmination calibration.
I have tried both ATR and manual sighting. I sight to the prism pivot point
The Leica GHM007 adjusts for slant height as far as I know
I have noticed this. And I have subtracted a few mm’s off the rod height.
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